1. Field of the Invention
The present invention generally relates to an extrusion molding equipment, and more particularly, to an extrusion molding equipment for making a multi-layer parison having a plurality of resin layers laminated therein.
2. Description of Prior Art
Conventionally, a hollow article of a resin material is generally formed through a blow molding process in which a pressurized gas is blown into a parison inserted into a set of molds. Sometimes a multi-layer parison having a plurality of resin layers laminated therein has been employed in the blow molding process when the properties for a molded product of a single resin material are insufficient for the intended use. In case of a fuel tank made of a resin material, such as high-density polyethylene, for example, the problem of gasoline permeation exists and the gasoline stored within the tank may permeate through the tank wall.
Japanese Patent Publication (Tokkosho) No. 58-23212 discloses a hollow article having a plurality of resin layers laminated therein. The article is formed from a multi-layer parison using known blow molding techniques as a way to deal with this permeation problem. More specifically, an extrusion molding process is used to produce the multi-layer parison. The multi-layer parison has an inner barrier resin layer of a nylon resin material for preventing the gasoline from permeating therethrough, an outer resin layer of high-density polyethylene, and an adhesive resin layer of a modified polyethylene. The adhesive resin layer is formed between the inner barrier resin layer and the outer resin layer and securely bonds the inner barrier resin layer to the outer resin layer. Accordingly, the hollow article is formed from the parison by the use of a known blow molding technique.
In conventional extrusion molding equipment as described above, each of the resin materials forming the three resin layers is stored in its own respective resin storing space. The multi-layer parison is formed by extruding the three resin materials simultaneously from a nozzle. However, when the aforementioned multi-layer parison is formed by conventional or known extrusion molding equipment, at least one resin material is extruded beyond the desired limits for that material. Therefore, the layer of resin material will extend beyond the desired limits. Also, difficulty is experienced in controlling the thickness of the layer as well as its cut off.
Accordingly, it is most important for each resin layer to be formed to the proper predetermined thickness and this requires appropriate control of the extrusion pressures of the resin materials, especially at the junction where the resin materials meet during the extrusion. This is generally accomplished in conventional extrusion molding equipment as described above by controlling each accumulator and extrusion piston and cylinders associated therewith by a time constant generation type control apparatus comprising relief valves, a proportional electromagnetic pressure control valve, a time constant generation type power amplifier and digital type predetermining equipment. Control of such a apparatus is intended to control properly the pressure at the junction of the resin materials. The extrusion operation of extruders and plungers is controlled by limit switches associated with the rods converted to the plungers in the extrusion cylinders. Also, starting times of extrusion of the resin materials are controlled by timers associated with the extrusion cylinders. In this manner delay times are set between the starting time of one resin material extrusion and the starting times of the other resin materials extrusion.
However, in the control system of the resin material pressure as described above, if resin is leaking from the nozzle, the resin material extrusion amount from the extrusion cylinder is more than desired. Therefore, it is difficult to control properly the resin material amount with the control system as noted. One may consider controlling the resin material extrusion amount by monitoring the resin pressure by the use of a pressure detecting device set to sense the resin material in the conduit of the extruder. However, the pressure in the conduit is changed according to the change of the resin material viscosity due to its temperature. Also, if the conduit becomes narrow by piling the resin material, the pressure in the conduit is changed to an abnormal value. Accordingly, proper control of the resin material amount by the control system described is difficult. These problems are particularly notable with respect to the extrusion of the inner layers of the barrier resin and the adhesive resin which are invisible from outside the parison.